Appl Microbiol Biotechnol (2003) 61:424–428 DOI 10.1007/s00253-003-1302-y

MINI-REVIEW

S. Sharma · P. Sachdeva · J. S. Virdi Emerging water-borne pathogens

Received: 1 October 2002 / Revised: 27 February 2003 / Accepted: 28 February 2003 / Published online: 9 April 2003 Springer-Verlag 2003

Abstract Emerging water-borne pathogens constitute a number of reviews and other publications available on the major health hazard in both developed and developing subject discuss emerging and re-emerging pathogens in nations. A new dimension to the global epidemiology of general, including water-borne, without addressing the cholera—an ancient scourge—was provided by the issues specifically pertinent to emerging water-borne emergence of Vibrio cholerae O139. Also, water-borne pathogens. This review critically examines which organ- enterohaemorrhagic Escherichia coli (E. coli O157:H7), isms really qualify as emerging water-borne pathogens, although regarded as a problem of the industrialized west, the possible reasons underlying their emergence and has recently caused outbreaks in Africa. Outbreaks of specific measures to face the challenge posed by them. chlorine-resistant Cryptosporidium have motivated water Due to the paucity of data, it may be difficult to decide authorities to reassess the adequacy of current water- which of all water-borne pathogens are emerging. Nev- quality regulations. Of late, a host of other organisms, ertheless, there are some clear-cut candidates. such as hepatitis viruses (including hepatitis E virus), No organism other than Vibrio cholerae could serve as Campylobacter jejuni, microsporidia, cyclospora, Yersin- a better example of an emerging water-borne pathogen. ia enterocolitica, calciviruses and environmental bacteria Cholera is an ancient scourge and to date seven like Mycobacterium spp, aeromonads, Legionella pneu- pandemics have been recorded. Of the several recognized mophila and multidrug-resistant Pseudomonas aerugi- serogroups, V. cholerae O1 has been responsible for these nosa have been associated with water-borne illnesses. pandemics. In 1992 however, a new strain called V. This review critically examines the potential of these as cholerae O139 Bengal appeared in South India and emerging water-borne pathogens. It also examines the caused explosive outbreaks of cholera-like disease (Ra- possible reasons, such as an increase in the number of mamurthy et al. 1993). In a matter of one year, the new immunocompromised individuals, urbanization and hor- strain spread to several parts of India and to neighbouring izontal gene transfer, that may underlie their emergence. Bangladesh and Thailand (Nair et al. 1994). By the end of Further, measures required to face the challenge posed by 1993, cholera outbreaks due to V. cholerae O139 were these pathogens are also discussed. reported from South Asia and other countries of the world. Soon after its appearance, V. cholerae O139 outnumbered V. cholerae O1 and became the dominant serogroup in India and other parts of the Indian sub- Emerging water-borne pathogens continent. It was thought that this was probably the beginning of a new pandemic—the eighth pandemic of In 1992, the Institute of Medicine described an emerging cholera (Nair et al. 1996). But, by 1994, there was a infection as any new, re-emerging, or drug-resistant dramatic decrease in V. cholerae O139 and once again V. infection whose incidence in humans has increased within cholerae O1 became the dominant species. It is thought the past two decades or whose incidence threatens to that, in the years to come, V. cholerae O139 is going to increase in the near future (Lederberg et al. 1992). A play an important role in the global epidemiology of cholera (Garg et al. 1998). S. Sharma · P. Sachdeva · J. S. Virdi ()) Following identification of the O139 serogroup and the Microbial Pathogenicity Laboratory, Department of Microbiology, finding that environmental, non-toxigenic strains may University of Delhi South Campus, play an important role in the evolution of toxigenic V. Benito Juarez Road, 110 021 New Delhi, India cholerae (Karaolis et al. 1995), there has been a lot of e-mail: [email protected] interest in the study of non-O1, non-O139 serogroups. At Tel.: +91-11-26879950 least three localized outbreaks of diarrhoea caused by Fax: +91-11-26885270 425 non-O1, non-O139 serogroups have been described in the enzie et al. 1994). The oocysts of Cryptosporidium are recent literature. These include an outbreak caused by V. resistant to the microbiocidal concentrations of chlorine cholerae O10 and O12 in February 1994 in Lima, Peru normally used for the disinfection of drinking water. Like (Dalsgaard et al. 1995), another caused by O10 in Delhi, E. coli O157:H7, Cryptosporidium has also been regarded India (Rudra et al. 1996) and an epidemic caused by as a problem in the developed countries. Nevertheless, the stable toxin producing non-O1 V. cholerae among organism has been isolated from the stools of diarrhoeic Khmers in a camp in Thailand (Bagchi et al. 1993). In patients in developing countries (Nath et al. 1999). 1996, an inexplicable upsurge in the incidence of non-O1, However, nothing is known about the prevalence of this non-O139 V. cholerae infections was observed among pathogen in the environmental waters of developing hospitalized patients in Kolkata, India, which even nations, which needs to be studied. Other enteric proto- outnumbered the O1 and O139 serogroups (Sharma et zoa, like microsporidia and cyclospora, have been al. 1998). Although non-O1, non-O139 V. cholerae are detected in surface, ground and treated wastewaters, not regarded as important enteropathogens currently, their indicating their potential as water-borne pathogens. They increasing incidence suggests their potential as important have been recognized as gastrointestinal pathogens with future water-borne pathogens. increasing frequency since the AIDS epidemic. However, Pathogenic Escherichia coli, such as enteroinvasive E. microsporidia have also been implicated as pathogens in coli, enteropathogenic E. coli, enteroaggregative E. coli otherwise healthy individuals (Goodgame 1996). Further and shiga toxin-producing E. coli (STEC), constitute a research is required to understand the distribution and very large and important group of water-borne pathogens. survival of microsporidia and cyclospora in aquatic STEC and particularly E. coli O157:H7 (also called as environments, so that their true potential as emerging enterohaemorrhagic E. coli or EHEC) have been associ- water-borne pathogens may be evaluated. ated clinically with presentations ranging from asymp- Campylobacters are becoming increasingly important tomatic infection to severe bloody diarrhoea, which may as the cause of acute gastroenteritis in both industrialized lead to life-threatening sequelae, such as haemolytic and developing nations. During recent years, an increas- uraemia syndrome. This organism, although primarily ing incidence of campylobacteriosis has been reported in associated with food-borne outbreaks, has also become an many developed countries (Shane 2000). Although important public health concern as a water-borne patho- mainly food-borne, water is also regarded as an important gen. Water-borne E. coli O157:H7 outbreaks due to route for the transmission of campylobacters. Most of the drinking water (Dev et al. 1991; Swerdlow et al. 1992) cases are sporadic, although large water-borne outbreaks and recreational water exposure have been reported have also been reported. Between 1992–1996, six (Brewster et al. 1994; Keene et al. 1994). A large outbreaks of campylobacteriosis occurred in Sweden epidemic of haemorrhagic colitis in Africa was also (Furtado et al. 1998). Two water-borne outbreaks of this reported to be a water-borne outbreak of E. coli O157:H7 organism were reported in Central Norway in 1994 and (Isaacson et al. 1993). Although generally considered to 1995. Recently, a water-borne Campylobacter jejuni be a problem in the developed nations, STEC have outbreak was reported from a Danish town due to recently been isolated from developing countries (Dutta et contamination of the water supply with ground water al. 2000; Khan et al. 2002a, 2002b). These isolates have (Engberg 1998). been obtained from either cattle or diarrhoeic human Yersinia enterocolitica, an important food- and water- subjects, both of which can act as sources for water-borne borne bacterium is known to cause a variety of gastro- E. coli O157:H7. The non-O157 verotoxin-producing intestinal problems. Most commonly, it causes acute strains are also transmitted by water (Chalmers et al. diarrhoea, terminal ileitis and mesenteric lymphadenitis. 2000) and need to be studied more intensively (Goldwater Post-infectious sequelae are manifested in the form of and Bettelheim 1998). Efficient methods to detect STEC, reactive arthritis. World-wide surveillance data on Y. including E. coli O157:H7, in water have been reported enterocolitica show great changes over the past two recently (De Boer and Heuvelink 2000), which would be decades and bring forth its emerging nature (Ostroff extremely useful in studying the prevalence of this 1995). The strains present in the aquatic environment are organism in water. extremely heterogeneous, belonging to biotype 1A. It has Cryptosporidium, a coccidian parasite, causes persis- been shown that biotype 1A strains of Y. enterocolitica tent diarrhoea (cryptosporidiosis) in immunocompro- may be pathogenic by some novel mechanisms. Thus, the mised individuals, particularly patients suffering from importance of Y. enterocolitica as an emerging water- acquired immunodeficiency syndrome (AIDS). Such borne pathogen needs to be assessed further. patients suffer from life-threatening infections due to this Several other microbial agents which may not un- pathogen. The organism is transmitted by ingestion of equivocally qualify as emerging water-borne pathogens water contaminated with the oocysts of Cryptosporidium nevertheless are potential candidates. These include and also by direct contact with the infected persons or several enteric viruses, environmental mycobacteria, animals. In the past, it has caused large water-borne aeromonads, Legionella pneumophila, Pseudomonas outbreaks (Krammer et al. 1996). The largest water-borne aeruginosa and calciviruses. Of the large number of outbreak due to Cryptosporidium was reported in 1993, in enteric viruses known, infections due to the hepatitis E Milwaukee, involving an estimated 400,000 cases (Mack- virus have definitely increased in the past few years. 426 However, it is difficult to decide whether this is a true host of environmental bacteria, namely Mycobacterium increase in its incidence, or whether our ability to detect spp, aeromonads, L. pneumophila and P. aeruginosa. The the virus more frequently now than previously is being newly originated category represents the truly new perceived as emergence. The epidemic hepatitis E affects pathogens exemplified by V. cholerae O139 and EHEC. mostly young adults between 20–40 years of age. A variety of reasons may underlie the emergence of However, children and old people are not immune to this newly recognized pathogens. An important reason, which disease. Calciviruses cause acute gastroenteritis. Al- is probably relevant to all, is the development of efficient though mainly food-borne, the importance of human detection methods, including molecular, immunological calciviruses as water-borne pathogens needs further and immunomagnetic techniques (Hurst and Toranzus investigation. The so-called environmental mycobacte- 1997). Another reason, which is more relevant to the ria—the Mycobacterium avium–intracellularae complex, emergence of Cryptosporidium, is the increase in the M. kansasi and M. fortuitum—are present in soil and number of immunocompromised persons, which is best water. They can cause infections of the skin, the lymph exemplified by patients receiving therapy for cancer or nodes and the respiratory and gastrointestinal tracts. organ transplantation, elderly individuals and patients Disseminated infections are seen only in immunocom- with AIDS. This also seems to be true for the emergence promised individuals. Although infections due to envi- of microsporidia. The emergence of these pathogens in ronmental mycobacteria have increased in the past the industrialized west is probably related to the relatively (Falkinham 1996), there is no evidence of the acquisition higher number of immunocompromised individuals in of infection by water. Aeromonads (Aeromonas sobria, A. this region. caviae, A. hydrophila) are widespread in surface waters Increasing urbanization, necessitating the use of vast and have been isolated regularly from drinking-water drinking-water distribution systems and the attendant distribution systems. Like environmental mycobacteria problems, has mainly been responsible for the emergence and aeromonads, L. pneumophila and P. aeruginosa can of water-borne Mycobacterium spp, aeromonads, L. also get into water distribution systems from source pneumophila and P. aeruginosa. waters. All these organisms have the ability to regrow in Of late, the ever-increasing movement of human such distribution systems. The ability of L. pneumophila beings from one part of the world to another may to grow inside fresh-water amoebae is noteworthy, introduce exotic pathogens into geographical areas where especially in hot- and cold-water distribution systems. the native population may have little immunity to them. The use of such contaminated waters for heating or Akin to this reasoning is the introduction of pathogens cooling towers results in the production of an aerosol of L. into newer geographical areas via the increasing interna- pneumophila which may be inhaled, leading to Legion- tional trade in food and foodstuffs. In fact, several food- naire’s disease—a form of severe pneumonia (States et al. borne pathogens are also transmitted by water. This may 1990). In contrast, P. aeruginosa is an opportunist par account for the introduction of European strains of Y. excellence (Lyczak et al. 2000). The importance of these enterocolitica into the American continent and vice versa organisms as water-borne pathogens is based primarily on witnessed in the past decade or so (Ostroff 1995). their ability to live in biofilms in water distribution Multidrug resistance has been responsible for the systems, where they can act as a continuous source of emergence or re-emergence of several pathogens, like M. contamination (Szewzyk et al. 2000). This is because tuberculosis, methicillin-resistant staphylococci, Neisser- organisms in biofilms differ considerably from their ia meningitides and enterococci. Among the emerging planktonic counterparts in terms of gene expression, water-borne pathogens however, multidrug-resistant P. metabolic activity and virulence characteristics aeruginosa is probably the only organism which qualifies (Morschhauser et al. 2000). for this category (Bert et al. 1998). The acquisition of virulence traits by horizontal gene transfer is responsible for the appearance of the truly new Emergence—possible causes pathogens. Evidence indicates this may be the cause for the emergence of V. cholerae O139 and E. coli O157:H7. The reasons underlying the emergence of water-borne Natural ecosystems like aquatic habitats contain diverse pathogens can, at best, be discussed in terms of possibil- microbial communities. These are characterized by the ities only. Unequivocal answers must await further presence of both resident (environmental/non-pathogenic) research. For a better understanding of the causes of microflora and faecally shed pathogenic forms from emergence, it may be advisable to divide emerging water- animal reservoirs or human patients. The environmental borne pathogens as newly recognized and newly origi- or non-pathogenic forms may serve as a storehouse of nated. The majority of the emerging water-borne patho- genetic determinants which, if transferred to other strains, gens belong to the former category, which means that, may confer novel virulence capabilities. Such inter- or although the etiologic agent was known for a long time, it intra-specific movement of genetic determinants may be was recognized only recently as the cause of water-borne mediated by bacteriophages, which are also an integral illness. This includes parasitic protozoa like Cryptospo- part of the aquatic ecosystems. Besides phages, transpo- ridium and Microsporidia, Campylobacter jejuni, several son-like elements, conjugative plasmids and integrans can viruses including calciviruses and hepatitis E virus and a also mediate similar transfer of genetic determinants. 427 Nucleotide analysis of the asd genes of 45 strains of V. measures suggested above pose a formidable task, a cholerae has yielded evidence which indicates that the beginning has already been made. This is evident from the classic and El Tor biotypes and the United States Gulf fact that much more is known about these pathogens Coast strains of V. cholerae O1 evolved independently today than five years ago. Studies on the molecular from environmental non-toxigenic, non-O1 strains epidemiology of V. cholerae O139 has opened new vistas (Karaolis et al. 1995). Therefore, it has become increas- on the aquatic ecology of water-borne pathogens, e.g. the ingly clear that the non-O1, non-O139 serogroups are role of autochthonous, non-pathogenic microbes in the involved in the emergence of newer variants of V. emergence of new pathogens and the role of zooplanktons cholerae, a fact supported by the genesis of V. cholerae and environment (especially temperature) in the spread of O139. This serogroup is believed to have evolved as a water-borne pathogens. Cryptosporidium and E. coli result of horizontal gene transfer between the O1 and the O157:H7 have brought to the fore how extremely low non-O1 serogroups (Bik et al.1995). However, genetic levels of certain pathogens in water may pose a serious evidence in respect of the emergence of E. coli O157:H7 threat to human health. The emergence of water-borne is more nebulous. The genotyping of E. coli O157:H7 has Cryptosporidium has motivated water authorities to shown that strains collected from geographically diverse reassess the adequacy of current water quality regulations areas are identical or nearly identical, indicating their (Gostin et al. 2000). Areas where further research is recent descent from an ancestral cell. These are related warranted include: the development of simple, quick and only distantly to other verotoxin-producing strains of E. foolproof methods for the detection of these pathogens in coli (Whittam et al.1998). Indirect evidence however water, their survival in aquatic habitats, their ability to indicates that genetic exchange among these is possible form biofilms in drinking-water distribution systems and (Bilge et al.1996) their resistance to disinfection. This is an enormous task Finally, the influence of sustained climatic changes on and requires dedicated inputs from researchers, public the emergence of water-borne pathogens also needs to be health officials and water authorities. considered. Both Cryptosporidium and E. coli O157:H7 were known to be present in cattle reservoirs before these Acknowledgements This work is supported by grants from the were recognized as water-borne human pathogens. 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